The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the Universal Mobile Telecommunication System (UMTS) and Long Term Evolution (LTE). The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative to UMTS.
In LTE, OFDM (Orthogonal Frequency Division Multiplexing) is used in the downlink. The LTE physical resource can be seen as a time-frequency grid, where each resource element, i.e. each square in the grid, corresponds to one OFDM subcarrier during one OFDM symbol interval. An LTE downlink subframe comprising 14 OFDM symbols (in case of normal cyclic prefix), with 3 OFDM symbols as control region, is illustrated in FIG. 1. Resource allocation in LTE is described in terms of resource blocks (RBs), and a subframe comprises a resource block pair, i.e. two time-consecutive resource blocks. The control region of a subframe comprises e.g. the Physical Downlink Control Channel (PDCCH), on which control information such as downlink scheduling assignments and uplink scheduling grants are transmitted. In the data region, data is transmitted on the Physical Downlink Shared Channel (PDSCH).
Some of the resource elements within the time-frequency grid are used to transmit reference symbols (RS), which are known symbols which may e.g. be used by the receiver for channel estimation in order to perform coherent demodulation. In LTE, cell specific reference symbols (CRS) are transmitted in all downlink subframes. They are also used for mobility measurements and for uplink power control performed by the UEs. Since the CRS is common to all UEs in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. As of LTE Release-10, a new RS concept was introduced with separate UE-specific RS for demodulation of PDSCH and separate RS for measuring the channel for the purpose of channel state information (CSI) feedback from the UE. The latter is referred to as CSI-RS. CSI-RS are not transmitted in every subframe, and they are generally sparser in time and frequency than RS used for demodulation. CSI-RS transmissions may occur every 5th, 10th, 20th, 40th, or 80th subframe according to an RRC configured periodicity parameter and an RRC configured subframe offset.
A detailed illustration of which resource elements within a resource block pair that may potentially be occupied by the new UE specific RS and CSI-RS is provided in FIG. 2.
There is an ever increasing demand for higher data rates in wireless networks, which poses challenges to developers of such networks. One approach to meeting requirements for higher data rates is to deploy heterogeneous networks (HetNets), i.e. a network containing nodes, e.g. base stations, operating with different transmission power. Base stations operating with high transmission power are herein denoted macro base stations, and base stations operating with lower transmission power are denoted low power nodes (LPN), but may also be referred to by other terms such as micro, pico, or femto base stations. The LPNs may further be stand-alone base stations, relays, or remote radio units (RRUs), also referred to as remote radio heads (RRHs).
Cell selection by wireless terminals is typically based on downlink (DL) received power, including the effects of the different base station transmission power. This leads to an ‘imbalance area’ surrounding the low power node where the path loss is lower towards the low power node, but the macro base station is still selected due to its higher transmission power. In the uplink (UL) direction, where the transmit power is the same, it would be better for a wireless terminal to be connected to the low power node also in this area. By increasing transmission power of the lower power node, the cell size of low power nodes can be increased. However, doing so affects the cost and size of the node, which in turn limits site availability. The range of the low power node can also be expanded by using a cell selection offset that favors the selection of the low power node. This leads to the UL signal being received in the best node, i.e. the low power node, and offloads the macro to a greater extent. These benefits, however, come at the cost of higher DL interference from the macro base station for users on the edge of the low power node cell.
Thus, solutions for inter-cell interference coordination (ICIC) are particularly important in heterogeneous networks. One approach is to separate transmissions from the macro layer and the pico layer in time, sometimes referred to as time-domain ICIC. This may be achieved by silencing the interfering macro base station in certain subframes. LTE Release 10 introduced Almost Blank Subframes (ABS), which are subframes with reduced transmit power (including no transmission) on some physical channels and/or reduced activity. The eNB may still transmit necessary control channels and physical signals as well as system information in the ABS, in order to ensure backwards compatibility toward UEs. Alternatively, the need to transmit these signals in ABS may be avoided by careful selection of ABS patterns.
Patterns based on ABSs may be signalled to the UE to restrict the UE measurement to specific subframes, called measurement resource restrictions. There are different patterns depending on the type of measured cell (serving or neighbour cell) and measurement type (e.g. RRM, RLM). One kind of pattern provides resource restrictions for CSI measurements of a primary cell (PCell). When this pattern is configured, two subframe subsets are configured per UE, and the UE reports CSI for each configured subframe subset. Typically, the two subframe subsets are chosen with the expectation that CSI measurements using the two configured subframe subsets are subject to different levels of interference. For example, one subframe subset may indicate ABSs while the second subframe subset indicates non-ABSs. For periodic CSI reports, linkage of each CSI report to a configured subset of subframes is defined in 3GPP TS 36.331, v.10.3.1. For aperiodic CSI reports, the UE reports CSI based on the subframe subset containing the CSI reference resource. Procedures for CSI reporting are described in 3GPP TS 36.213, v10.3.0, in particular in sections 7.1.2 and 7.1.3.
When transmitting data in Almost Blank Subframes, the general understanding has been that the network node should use zero power, i.e. “no transmission”. However, the reduced transmission time degrades performance for users connected to the macro base station, and leads to lower data rates. System performance evaluations have recently shown that reduced, but non-zero, power on data transmissions in ABS could provide significant performance improvements over zero-power transmissions, in particular for unicast transmissions, i.e. transmissions directed to a single user.
By transmitting unicast PDCCH and corresponding PDSCH or Physical HARQ Indicator Channel (PHICH) with reduced power in ABS, the flexibility of the macro scheduler may increase significantly. For example, macro users with good radio coverage could also be scheduled in ABS, but with reduced power data transmissions, and thus release resources/capacity for coverage limited macro users that would need the high power downlink subframes to receive data with an acceptable throughput. Another example is the possibility to transmit grants in ABS to enable uplink transmissions for macro users closer to the macro eNB which otherwise would be restricted to receive grants in non-ABS only. As these users are likely operating rather close to the macro eNB, the uplink interference towards pico eNBs would be limited. The reduction of the transmit power on unicast data physical channels in ABS would typically be in the order of magnitude of the largest configured cell selection offset within the macro coverage area.
In order to demodulate data transmissions based on higher order modulation schemes as well as deriving CSI feedback correctly, the transmit power differences between resources carrying PDSCH and reference signals need to be known by the UE. Therefore, the differences of the PDSCH transmitted energy per resource element (EPRE) to CRS EPRE and of PDSCH EPRE to CSI-RS EPRE can be derived by the UE from parameters provided by higher layer signaling, in which a UE may assume the EPRE of the reference signals to be constant across the downlink system bandwidth and constant across all subframes. In current specifications, these signaled power differences are assumed to be valid for all subframes, which clearly would not be applicable in the case of reduced power on data transmissions in ABS. Hence, in order to support reduced power on unicast physical channels in ABS the UE needs to be able to derive transmit power offsets between PDSCH and CRS/CSI-RS that are valid in certain subframes only, as illustrated in FIG. 3.
It has therefore been proposed in 3GPP to add additional signaling for supporting different unicast data transmit power offsets of PDSCH with respect to CRS/CSI-RS in ABS and non-ABS subframes, respectively.
Coordinated multi-point (CoMP) transmission and reception is being considered for LTE-Advanced Rel. 11 as a tool to improve the coverage of high data rates, the cell-edge throughput, and also to increase system throughput. CoMP implies dynamic coordination among multiple geographically separated transmission and/or reception points, where a “point” refers to a set of geographically co-located antennas. FIG. 4 shows an example CoMP heterogeneous network scenario with a macro base station equipped with low-power remote radio heads (RRHs) forming pico cells. The transmission/reception points created by the RRHs have different cell IDs than the macro cell. Typically, the RRHs will be connected to the macro base station via fiber or other fast backhaul connection in order to enable the coordination of transmission and reception.
There is room for further enhancements with respect to the transmission of reduced-power subframes, in particular when network deployments such as the one illustrated in FIG. 4 are considered.